1. Field of the Invention
This invention is concerned with the conversion of a methanol feed to a hydrocarbon mixture of high ethylene content in the presence of an improved aluminosilicate zeolite catalyst. More particularly, this invention relates to a process for the conversion of methanol to ethylene plus gasoline in the presence of a specified crystalline aluminosilicate zeolite catalyst characterized by a crystal size of at least about 1 micron.
2. Description of the Prior Art
The increasing demand for inexpensive petrochemical raw materials such as ethylene has spurred efforts to provide efficient means for manufacturing ethylene from raw materials other than petroleum. Ethylene is a primary material source used in the manufacture of polyethylene and styrene monomers which are important to the production of synthetic fibers, plastics and petrochemicals. The principle raw material for ethylene at the present time is petroleum naphtha, which is steam cracked to produce a mixture of products from which ethylene is recovered.
Another source for the production of ethylene is methanol which is converted to olefins, including ethylene, and gasoline hydrocarbons in the presence of an aluminosilicate zeolite catalyst, such as ZSM-5. The factors which improve ethylene selectivity in the co-production of ethylene plus gasoline from methanol over zeolite catalysts include low diffusivity, low methanol partial pressure, dilution of the feed stream and partial methanol conversion.
For example, the highest ethylene selectivities in the co-production of ethylene plus gasoline from methanol over zeolite catalysts are achieved in the presence of small-pore crystalline aluminosilicate zeolites such as erionite and ZSM-34. Examples of this form of catalytic conversion can be found in U.S. Pat. Nos. 4,062,905 which discloses the use of erionite and 4,079,096 which discloses methanol conversion over ZSM-34. U.S. Pat. No. 4,062,905, also contains a broad teaching that catalysts used in the conversion of methanol can be subjected to thermal treatment, including steaming. It has been found, however, that these small-pore zeolites have the disadvantage of converting up to 15 to 20 percent of the methanol charge into coke. Additionally, the cycle times of these catalysts are only a few hours.
Another method of improving ethylene selectivity is to conduct the methanol conversion at sub-atmospheric partial pressure of reactant feed over an HZSM-5 as disclosed in U.S. Pat. No. 4,025,575.
Further methods of improving ethylene selectivity in the conversion of methanol over zeolite catalysts involve dilution of the reactant feed. U.S. Pat. No. 4,079,095 discloses the conversion of methanol and water in the presence of a catalyst comprising a crystalline aluminosilicate zeolite of the erionite-offretite family to a hydrocarbon product rich in ethylene and propylene. U.S. Pat. No. 4,083,888 discloses a process of converting a methanol feed to a hydrocarbon mixture of high ethylene content by catalytic contact of the feed in the presence of a substantially anhydrous diluent such as hydrogen, helium, nitrogen, carbon dioxide, lower alkanes and flue gas. The catalyst used is a crystalline aluminosilicate zeolite such as HZSM-5 having a crystallite size greater than about 0.5 micron. U.S. Pat. No. 4,083,889 describes a process of catalytic conversion of methanol in the presence of steam or water diluent over a ZSM-5 type catalyst in order to enhance ethylene production. The presence of the steam diluent induces sustained high catalytic activity with high selectivity for the formation of ethylene even at high conversion levels.
The diffusional characteristics of crystalline aluminosilicate zeolites can be moderated to increase ethylene yields upon the catalytic conversion of methanol. For example, U.S. Pat. No. 4,148,835 discloses a process for the catalytic conversion of methanol to light olefinic hydrocarbons with high selectivity for ethylene production by utilizing a crystalline aluminosilicate zeolite having a crystal size of at least about 1 micron. It has also been observed that silica-modified HZSM-5 also produces unusually high ethylene yields. The deposited silica within the pore structure of the zeolite limits diffusivity and results in an increase in ethylene selectivity compared to the parent zeolite. The intrasilica large crystal zeolite is somewhat less active than the parent zeolite which also is attributed to the silica "stuffing". However, even zeolites of large crystal size and the intrasilica version thereof suffer a serious deficiency in processing. Both have cycle times of only about 7 to 10 days, and require substantial daily increases in temperature, about 10.degree. F. per day, to maintain a constant conversion of between about 50 and 60 percent.
U.S. Pat. No. 4,118,431 discloses a process for the conversion of methanol to gasoline boiling components in the presence of a catalyst in which the active catalyst particles are mixed with heat absorbing inert particles such that the alpha activity of the combined solids in the reactor falls in the range of about 20 to 50. The ratio of catalyst particles to heat absorbing inert particles is varied as a function of the catalyst activity and in an arrangement to particularly limit undesired exposure of the catalyst downstream of the reaction front to deactivation of catalyst particles by process formed steam. The addition of the inert solids to the process are disadvantageous in view of the increased handling problems and increased complexities which are added to the methanol conversion process.